1. Field of the Invention
The present invention relates to a pump device, in which in a variable capacity pump the pressure and discharge volume of oil are increased gradually in accordance with a value required by an engine or hydraulic equipment, and the load acting on the pump, the engine and so forth can be kept to a minimum.
2. Description of the Related Art
The theoretical discharge volume of a gear pump is determined ordinarily by, among other factors, tooth length and tooth width, and the discharge volume is determined by the theoretical discharge volume and the rotational speed of the gears (pump revolutions). In a case where such a gear pump is used, for instance, as an oil pump for supplying lubricating oil into an engine for vehicles, the theoretical discharge volume of the oil pump is set in such a manner that the necessary amount of oil can be supplied also when the output of the engine, as a driving source, is low and pump revolutions are low.
When pump revolutions increase accompanying higher engine output, on the other hand, an excessive amount of oil, beyond the required amount, may in some instances be supplied to the engine, and the oil pump may consume thus substantial driving force, which may result engine output loss. Known gear pumps that solve the above problem include variable-capacity gear pumps in which either a drive gear or a driven gear, or both, moves in the axial direction as pump revolutions increase, so that a meshing area decreases as a result, and the theoretical discharge volume is reduced accordingly.
Conventional external gear pumps have been disclosed wherein a driven gear moves in the axial direction, whereby a meshing area (axial-direction height) is modified; as a result, the theoretical discharge volume varies proportionally to the meshing area between a drive gear and the driven gear. One such pump is disclosed in JP-T-2007-514097. An overview of the features in JP-T-2007-514097 follows next. The reference numerals of members in the following explanation are as used in JP-T-2007-514097. Specifically, the external gear pump of JP-T-2007-514097, as illustrated in FIG. 1 of that document, comprises a first conveying gear 5 (drive gear) and a second conveying gear 6 (driven gear).
A spring piston 9 is disposed on the left of the second conveying gear and a pressure piston 8 is disposed on the right. The second conveying gear is coupled to the pistons on both sides, by way of a journal bolt 7, to form a displacement unit 10. The meshing area of the conveying gears 5 and 6 is modified, and the pump conveyance volume is likewise modified, through displacement of the displacement unit 10 in the axial direction. The displacement of the displacement unit 10 in the axial direction depends on an external force that acts on the displacement unit 10.
That external pressure is in the form of operational oil pressure, supplied to a chamber 11, and which acts on the pressure piston 8. The force from a reset spring 12, as well as control pressure from the control piston 1 and that is supplied to a spring chamber 13, act also thereon. In a working example of FIG. 5 of JP-T-2007-514097, a control piston 1 of FIG. 1 of this patent document is arranged inside a displacement unit 60.
In FIG. 5 of JP-T-2007-514097, an electromagnetic valve 93 is disposed in a conduit 92 that supplies operational oil pressure in a chamber 66 on a side of the displacement unit 60 opposite to the side at which a reset spring 67 is present. The electromagnetic valve 93 closes upon a rise in the operational oil pressure as given by an engine control device. At the same time, the pressure in a chamber 66 is reduced via a connection piece 94. As a result of the rise in the operational oil pressure, the reset spring 67 causes the displacement unit 60 to move to a position of highest conveyance volume.
Herein, the operational oil pressure in the chamber 66 on a side of the displacement unit 60 opposite to the side at which the reset spring 67 is present corresponds to the oil pressure exerted through switching of the electromagnetic valve 93, or to a reduction of the pressure in the chamber 66, via the connection piece 94, through closing of the electromagnetic valve 93. In such a configuration, however, the only control that is possible is between a state in which oil pressure is acting, and a state in which it is not. Therefore, the extent by which the displacement unit 60 slides in the axial direction cannot be controlled finely over multiple stages.
As a result, the displacement unit 60 cannot be displaced to a slide position at which a discharge volume and oil pressure are generated in accordance with the oil discharge volume and oil pressure that are required by the engine or hydraulic equipment, in various revolution ranges. Also, an oil discharge volume and oil pressure that are equal to or greater than required are generated in a given revolution range. This results in inefficient changeover.